Quenching of metals



Patented Feb. 1, 194-4 unuuu nu LHLHUL,

UNITED STATES PATENT OFFICE QUENCHING OF METALS Delaware No Drawing.Original application January 12,

1942, Serial No. 426,536. Divided and this application July 13, 1943,Serial No. 494,531

3 Claims.

This invention relates to the quenching of metals; and it isparticularly concerned with improved quenching oil compositionscomprising a light mineral quenching oil and dried rubber latex in acontrolled amount adequate to produce a quenching oil composition havingan initial five second quenching speed of at least about 22.0 per centwithout substantially modifying the stress-reducing characteristics ofthe oil; and it further comprises an improved method of quenching metalswherein the preheated metal is quenched by immersion in a quenching oilbath consisting essentially of a mineral quenching oil and a controlledamount of dried rubber latex adequate to impart to said bath an initialflve second quenching speed of at least about 22.0 per cent withoutsubstantially modifying the stress-reducing characteristics of the oil;all as more fully hereinafter set forth and as claimed.

While our invention is useful in the quenching of any metal which isadvantageously quenched in an oil bath having a high initial quenchingspeed, it is particularly useful in the quenching of iron base alloysand it will be described hereinafter in detail in connection with thequenching of steel.

Many metal alloys, particularly iron base alloys, such as carbon steelsand alloy steels, require heat treatment for the development of maximumproperties of hardness and strength. These properties are dependent uponthe establishment of certain physical structures in the metalliccomponents of the alloy. In steel, hardenability is determined by theextent to which a martensitic structure is established in the alloy. Theproduction of this structure in steel is usually accomplished byarresting at the desired point, the changes in the internal structure ofthe alloy which take place during the cooling of the steel from hightemperatures. The fact that these physical changes require time fortheir completion makes it possible to arrest them at the desired pointby suitable quick cooling.

Quenching of the steel in aqueous or oil quenching baths is generallyused to arrest these physical changes. It is advantageously carried outin such manner that the physical changes in the steel are arrested at ornear the point at which maximum hardness is obtained, and is thenfollowed by a tempering treatment involving heating at relatively lowtemperatures to impart the desired ductility or toughness to the metalat some sacrifice in hardness.

Quenching in aqueous quenching media develops the properties of strengthand hardness to the maximum obtainable for a given section of metal.However, quenching in aqueous quenching media is undesirable in manycases because these media tend to set up excessive amounts of internalstress in the steel resulting in distortion and warping and, in theextreme case, cracking of the quenched piece. As a result, aqueousquenching media have been supplanted by mineral oil quenching mediawhere such mineral oil quenching media have sufficiently high quenchingspeeds to produce the desired properties in the quenched metal becausesuch mineral oil quenching media are particularly adapted to minimizeinternal stresses and distortion in the quenched product. This resultsfrom the fact that the quenching speeds of mineral oil quenching mediaare substantially slower than those of aqueous quenching media, due toconsiderable extent to the formation of a more or less persistent vaporenvelop about the quenched piece at the beginning of the quenchingcycle, and the fact that in the later stages of the quenching cycle thequenching speeds of mineral oil quench ing media are sufliciently slowso that the internal stresses developed in the metal in the early stagesof the cycle tend to be relieved. However, since the overall coolingefficiency of the mineral oil quenching media heretofore known is not asgreat as that of aqueous quenching media, it has been difllcult orimpossible to effect in mineral oil quenching baths quenching of piecesformed of steels having high critical cooling rates, sufiicientlyrapidly to develop the properties of strength and hardness to theirmaximum values, or the quenching of pieces having high mass to surfaceratios, sufliciently rapidly to develop the properties of hardness andstrength to the maximum depth.

It is desirable, therefore, to improve the quenching of metals inmineral oil quenching media so that the cooling efiiciency in the earlystages of the quenching cycle is increased to more nearly approach thecooling efiiciency of aqueous quenching media without increasing thequench- -ing speed in the later stages of the cycle suf- OUHI'UH HUD"!ficiently to deleteriously affect the normal stressrelieving propertiesof the mineral oil.

It is an object achieved by the present invention to provide a method ofquenching metals, particularly iron base alloys, in mineral oilquenching media having quenching speeds greater than the mineral oilquenching media heretofore known. It is a further object achieved by thepresent invention to provide mineral quenching oil compositions havingcooling efllciencies more nearly approaching the cooling eificiency ofwater while retaining the stress-preventing characteristics of oilquenches.

The critical cooling rate of a metal is defined as the lowest coolingrate at which maximum hardness is developed. For steel the criticalcooling rate is the lowest cooling rate which will produce a fullmartensitic structure in the steel. Thus it is generally most desirableto effect initial quenching at a speed sufiicient to produce a coolingrate at least equal to the critical cooling rate for as great a depth inthe metal as possible and to eifect the quenching with a minimum ofdistortion.

The cooling eiiiciency of a quenching medium is customarily defined interms of quenching speed and the initial five second quenching speed isa convenient measure of this property of quenching oil compositions, andis the property with which this invention is most concerned. The initialfive second quenching speed of a quenching oil composition as referredto throughout this specification and in the appended claims isdetermined according to the following formula:

Temperature rise in 5 sec. test Temperature rise in cold quench test Percent available heat removed m ilrst five seconds furnace equipped withan automatic temperature control. Two quarts of the quenching oilcomposition to be tested are placed in a calorimeter which has a wirescreen positioned in the center of the oil bath to insure constant depthof immersion of the samples. position is heated to an initialtemperature of 100 F.

One of the preheated test pieces is immersed in the quenching bath bymeans of light tongs for a period of five seconds without agitation. Thetest piece is then removed and the bath agitated and the maximumtemperature reached is measured. A second preheated test piece isimmersed in the bath at an initial temperature of 100 F. and the bath isagitated until the maximum rise in temperature has been produced. Themaximum temperature rise measured in this cold quench represents theavailable heat of the test piece. By substituting the values thusdetermined for the five second quench and for the cold quench in theabove formula a measure of the heat removed in the first five seconds ascompared with the total available heat is obtained and is referred toherein as the initial five second quenching speed.

We have discovered that dried rubber latex has the property ofincreasing the initial five second quenching speed of mineral oilswithout lessening their stress-reducing characteristics, and that byincorporating it in suitable amounts in light The quenching commineraloils of the type heretofore used in quenching metals, we can producequenching media having initial five second quenching speeds of at least20.0 per cent, while retaining the distortion-minimizing characteristicsof mineral oil quenching media. We have found further that by quenchingpieces formed of metals having high critical cooling rates or piecesdifiicult to quench because of a high ratio of mass to surface, byimmersion in quenching oil compositions containing dried rubber latex,we can eilect cooling of the metal at a rate more nearly approaching orequal to the critical cooling rate of the metal without creatingexcessive stresses in the metal. In this way we have been able todevelop the hardness of metals having high critical cooling rates to adegree heretofore unattainable with oil quenching media and have beenable to effect hardening to a greater depth in pieces having high ratiosof mass to surface, and have achieved both results with a minimum ofdistortion.

Dried rubber latex incorporated in a mineral quenching oil in very smallamounts produces quenching compositions having reasonably good quenchingspeeds. Amounts between about 0.25 and 2.0 per cent by weight producequenching media having initial five second quenching speeds betweenabout 22.0 and 28.0 per cent, the highest values being obtained at about1.0 per cent by weight. The effect of this material in increasing theinitial quenching speed of the oil increases directly with the amountadded up to a maximum and then decreases.

In compounding the quenching oil compositions of our invention we selectfor the quenching bath a mineral oil of the type customarily used forquenching steel and other metals. The oils used for this purpose may beeither naphthenic or parafiinic oils and are usually acid treatedneutral oils having a viscosity of about 70 to 200 S. U. V. at F.,relatively high flash and fire points, and substantial heat stabilityand resistance to sludging. For optimum results we have found thatpaiaffinic oils which usually have an initial five second quenchingspeed between about 17.0 and about 19.0 per cent are most advantageous.

The best results are obtained according to our invention byincorporating in the oil an amount of dried rubber latex sufiicient toproduce a quenching oil composition having an initial five secondquenching speed which can eifect cooling of the metal to be quenched ata rate approaching the rate obtainable with aqueous quenching media. Inthe case of many steels, initial five second quenching speeds of atleast about 22.0 per cent are required for this purpose, andparticularly for the quenching of many pieces having high mass tosurface ratios these high initial five second quenching speeds areparticularly advantageous.

The actual quenching operation using the quenching compositions of ourinvention is usually carried out by immersion of the preheated metal inthe quenching oil composition until sufficient heat has been removed toreduce its temperature the desired amount. The quenching oil may be usedrepeatedly with only slight deterioration.

In the following example there is illustrated specifically the resultsobtainable with several concentrations of dried rubber latex used inaccordance with the process of our invention.

A series of quenching oil compositions was made by using a neutralparafllnic mineral oil cause litl LlitllUt having a viscosity of 100 S.U. V. at 100 F. and varying amounts of dried rubber latex between 0.5and 1.5 per cent. The initial five second quenching speeds of therespective compositions were then determined as above described and thefollowing results were obtained:

Quenching speed Quenching oil No. 2 16.98 Quenching oil No. 2+0.5% latex26.54 Quenching oil No. 2+1.0% latex 27.78 Quenching oil No. 2+1.5 latex25.16

It will be observed from the results of this test that the optimumconcentration of dried rubber latex is about 1.0 per cent and that allconcentrations between 0.5 and 1.5 per cent produce quenching oilcompositions having initial to such specific embodiments except ashereinafter defined in the appended claims.

What we claim is:

1. An improved quenching oil composition comprising a mineral quenchingoil and dried rubber latex .in controlled amount adequate to impart tosaid quenching oil composition an initial five second quenching speed ofat least about 22.0 per cent without substantially modifying thestress-reducing characteristics of the oil.

2. An improved quenching oil composition comprising a light mineralquenching oil and 0.25 to 2.0 per cent by weight of dried rubber latex.

3. A method of quenching metals comprising heating the metal to atemperature above its critical temperature and thereafter cooling themetal by immersion in a quenching oil bath comprising a light mineraloil and a controlled amount of dried rubber latex adequate to impart tothe quenching oil bath an initial five second quenching speed of atleast about 22.0 per cent.

CLIFFORD G. ZUR HORST. BLAINE B. WESCOTT. LESLIE W. VOLLMER.

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